Simulating the Colocation of High-Precision Microwave and Optical Techniques for Tropospheric Parameter Determination in Context of the ACES Mission.

The future Atomic Clock Ensemble in Space (ACES) mission of the European Space Agency (ESA) will address the development of a time transfer concepts for tomorrow's technologies. The ACES configuration includes a new generation of high-precision atomic clocks, a microwave link terminal (MWL) on the ground and on the satellite, and an optical detector and reflector also on the satellite. Due to the fact, that the official launch date is in 2025, there is a lack of real observation data. For that, a full-scale simulation software has been implemented. The simulator produces MWL code and phase observations in downlink and uplink, as well as one- and two-way laser observations. To analyse the efficiency of a time transfer concept before launch, we used the simulator to generate a data set of 100 passes during July 2021.
Investigations based on this data set showed that the colocation of the high-precision geodetic observation techniques of the ACES mission could better separate the individual error contributions of a measurement. Due to the colocation of optical and microwave-based geodetic observation techniques, also error parameters like orbit and troposphere correction can be estimated together. Estimation of a common troposphere for all observation techniques, improves the accuracy of the determination of the offset between ground and ACES clocks. Our further investigations focus on the common troposphere estimation of multi-color optical observations, together with microwave-based observations and the effects of different weighting methods. An extension to a network of ground stations will demonstrate the advantages of the ACES mission for synchronizing multiple ground clocks. The colocation of different high-precision geodetic observation techniques and estimation of common parameters will benefit timing and ranging applications and fundamental physics studies.